Tropical forests leak nitrogen back into atmosphere, say scientists

May 22, 2006

Princeton, N.J. - In findings that could influence our understanding of climate change, a Princeton research team has learned that tropical forests return to the atmosphere up to half the nitrogen they receive each year, thanks to a particular type of bacteria that lives in those forests.

The bacteria, referred to as "denitrifiers," exist in forest soil, where they live by converting the nitrates fed upon by tree roots back into nitrogen gas, which is lost to the atmosphere. The researchers who recently discovered this behavior say the findings are important for our understanding of how tropical forests fit into the earth's climate system.

"Tropical forests play a major role in regulating the planet's climate, and these findings indicate that we are still working on our basic understanding of the nitrogen cycle," said Lars Hedin, a researcher on the team and professor of ecology and evolutionary biology at Princeton. "That a group of bacteria can have such a dramatic impact on forest nutrition debunks our previous theories about how nitrogen behaves in forests, and shows us that these microorganisms affect soil nutrients and forest growth."
-end-
The team, which also includes first author Benjamin Houlton, a student from Hedin's lab now doing postdoctoral work at Stanford, and Daniel Sigman, a Princeton professor of geosciences, will publish their findings in the May 22 issue of the journal, Proceedings of the National Academy of Sciences. Hedin is available for comment at (609) 558-9096.

ABSTRACT

Isotopic evidence for large gaseous nitrogen losses from tropical rainforests

Benjamin Z. Houlton, Daniel M. Sigman, and Lars O. Hedin

The nitrogen isotopic composition (15N 14N) of forested ecosystems varies systematically worldwide. In tropical forests, which are elevated in 15N relative to temperate biomes, a decrease in ecosystem 15N 14N with increasing rainfall has been reported but never conclusively explained. This trend is captured in a set of well characterized Hawaiian rainforests, across which we have measured the 15N 14N of inputs and hydrologic losses. We report that the two most widely purported mechanisms, an isotopic shift in N inputs or isotopic discrimination by leaching, fail to explain this climate-dependent trend in 15N 14N ratios. Rather, isotopic discrimination by microbial denitrification appears to be the major determinant of N isotopic variations across differences in rainfall. In the driest climates, the 15N 14N of total dissolved outputs is higher than that of inputs, which can only be explained by a 14N-rich gas loss. In contrast, in the wettest climates, evidence indicates that denitrification completely consumes nitrate in local soil environments, thus preventing the expression of its isotope effect at the ecosystem scale. Under these conditions, the 15N 14N ratios of bulk soils and stream outputs decrease to converge on the low 15N 14N ratios of N inputs. N isotope budgets that account for such local isotopic underexpression suggest that denitrification is responsible for a large fraction (24-53%) of total ecosystem N loss across the sampled range in rainfall.

Keywords:
climate isotope tropics

Princeton University

Related Climate Change Articles from Brightsurf:

Are climate scientists being too cautious when linking extreme weather to climate change?
Climate science has focused on avoiding false alarms when linking extreme events to climate change.

Mysterious climate change
New research findings underline the crucial role that sea ice throughout the Southern Ocean played for atmospheric CO2 in times of rapid climate change in the past.

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.

Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.

Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.

Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.

A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).

Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.

Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.

Read More: Climate Change News and Climate Change Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.